Digital image printing systems which use multiple ink nozzles integrated within a print head have increased rapidly in popularity in recent years. Either at manufacture, or during operation, the particular printing nozzles within a print head can fail to deliver ink droplets of the required size, or alternatively, with the required positional accuracy. These problems can arise from a combination of manufacturing flaws, and wear and tear. In extreme cases, individual nozzles can fail to deliver any ink whatsoever, due to being blocked or damaged.
Defective nozzles, be they blocked or merely defective in terms of their performance, can be identified manually or automatically by examining test print output.
Even where nozzles delivery ink droplets satisfactorily, the nozzles can vary in their characteristics, and produce uneven print densities. U.S. Pat. No. 5,038,208 entitled “Image Forming Apparatus with a Function for Correcting Recording Density Unevenness” describes a method and apparatus related to improving the evenness of print density produced by nozzles of varying characteristics. The patent discloses a printer having a multi-nozzle printing head, this printer also storing data associated with the image-forming characteristics of each of the multi-nozzle heads. Furthermore, a corrector means for correcting the image forming signals based upon the data stored in memory corresponding to the multi-nozzle head characteristics is disclosed. The aforementioned correction is performed either by retrieving a corrected value from a memory, or by retrieving a correction value from a memory, and adding the correction value to the uncorrected image value. In both cases, the image correction data is accessed using both the uncorrected image value, and a nozzle counter value. This type of correction is known as “head shading”. Head shading methods do not, however, satisfactorily eliminate print artefacts where a blocked nozzle is present, or where the ink ejection performance of a nozzle fails to meet minimum requirements.